U.S. Pat. No. 3,925,668 discloses a temperature measuring device designed to remotely measure the temperature of an object, i.e., the temperature is measured without direct contact with the object. The device is useful in various clinical and industrial applications and for various consumer uses. Radiant energy from the object being monitored is sensed by the device by use of an aperture, optical filter, electro-mechanical modulator and a radiation sensor along with appropriate electronic circuitry and a temperature indicator output display. The modulator for the foregoing device .consists of a magnet, a moving coil and a vibrating needle disposed in the path of the radiation, which is adapted to change the radiation impinging upon the sensor from a steady state to a pulsed state.
U.S. Pat. No. 4,233,512 also discloses a thermometer for making remote temperature measurements, including a temperature sensor element and a modulator for converting radiation emitted by the object being monitored from a steady state to a pulsed state. The modulator consists of a piezoelectric ceramic reed element adapted to chop the radiation at a precise, fixed frequency prior to impingement on the sensor element. The piezoelectric device forms part of a resonant circuit in which the device serves as an active element in a phase locked loop arrangement used to drive and stabilize the modulator. The piezoelectric device deflects under applied voltage and also generates a signal that is used to lock the circuit onto the resonant frequency of the device.
One object of the present invention is to provide a remote temperature measuring device having a modulator with improved means for generating a feed back signal for phase lock control of the modulator. Another object of the invention is to extend the frequency range under which the modulator can be operated to provide useful resonant motion.